[otrs] Discard follow-ups to closed tickets (in addition to "rejected" auto responses)

Hello list members, is it possible to configure OTRS 5 to discard all follow-ups sent by customers for tickets that have been closed? I have configured the Raw queue to reject follow-ups, and customers do receive "message has been rejected" auto responses, but follow-up messages for closed tickets are nonetheless added to the OTRS database and notifications are sent to the ticket owners. I have been searching the Net for a long time without success, so I thought it best to ask here. -Ralph